Nanoscale study of the electrical properties of semiconducting polymers

Unlike traditional electronics, organic electronics uses small molecules or polymers with the desired electronic properties. The electrical conductivity of (semi-)conductive polymers depends on their dopant concentration. One of the advantages of organic electronics is its low cost, and the ability to produce flexible devices.

Organic semiconductors have been extensively studied for photovoltaic applications in recent decades. One of the objectives is to optimize the photon-to-energy conversion efficiency of photovoltaic devices.

The latter requires charge carrier mobility and appropriate microscopic morphology. In addition, each material must present efficient electrical conduction paths to the electrode under consideration to collect the maximum number of carriers. Regioregular poly(3-hexylthiophene) (P3HT) is widely studied in the field of organic electronics. With very high regioregularity, this polymer is known to form nanofibrils. This semi-crystalline morphology is ideal for charge transport. 
For photovoltaic devices, the introduction into active films of carbon nanotubes (CNTs), acting as conduction paths to the electrodes, is of particular interest as they are characterized by very high charge carrier mobility, enabling the electrical percolation threshold to be reached with only a small amount.

The particular morphology of P3HT fibers is highly effective in bringing the active material's charges to the nanotubes.

Electrical property mapping in P3HT:CNT mixtures can be measured by AFM in conductive mode. This is based on simultaneous measurements of the topography and the current flowing between the tip and the sample.

Atomic Force Microscopy (AFM) is a technique for visualizing the three-dimensional morphology of a material's surface with nanometric resolution, and for mapping certain properties (adhesive, mechanical, magnetic, electrical, etc.). The technique can be used to observe the surfaces of all types of solid materials (polymers, powders, glass, textiles, fibers, biological samples, nanoparticles, etc.) in air and in liquids at atmospheric pressure.

AFM can be used to image the surfaces of all types of solid materials and their physical properties at the nanometric scale.

The AFM principle is based on the measurement of the various interaction forces (ionic repulsion forces, Van-der-Waals forces, electrostatic forces, etc.) between the atoms on the surface of the sample to be observed and the atoms of a nanometric probe tip, fixed under a flexible microlever. A laser beam, reflected off the back of the microlever, is directed onto a 4-quadrant photodiode. The tip scans the surface and follows the topography of the sample, giving a three-dimensional image of the material being analyzed. This image is particularly useful for calculating roughness parameters.

Different types of probes can be used to qualify and quantify the various physical properties of the surface.

The AFM's Conductive mode (C-AFM) enables simultaneous acquisition of the 3D topography and current mapping of a surface. A potential difference (VDC) is applied between sample and ground (or tip and ground) to measure the current flowing from the tip to the sample.

TESCAN ANALYTICS has over 30 years' expertise in the use of AFM and its different modes on all types of materials. With state-of-the-art instruments, our team of experts works with all industrial sectors.

Figure 1: C-AFM 100 x 22 µm² topography (A) and current (B) images of a P3HT deposit (VDC = -500 mV). (C) Current profile along the white line in image (B). 7.6 x 3.7 µm² zooms of height (D) and current (E) from images (A) and (B), respectively.


Figure 2: AFM tapping mode 1.5 x 1.5 µm² height (A) and phase (B) images of a P3HT: CNT mixture on a glass/ITO substrate; the vertical color code is 9 nm for image (A) and 20° for image (B); the inset in image (A) is a 250 x 250 nm² zoom and that in image (B) is a 400 x 400 nm² zoom with the carbon nanotube highlighted (red dotted line).


Figure 3: (A) C-AFM image of 3 x 3 µm² current of a P3HT:CNT mixture on a glass/ITO substrate. The current scale is 250 pA (-125 (red) to +125 pA (green)) and the DC bias of the sample ranges from 500 mV (bottom of image) to 750 mV (top of image). (B) I-V curves derived from the C-AFM image.
 

Figure 1 shows topographic and electrical images obtained simultaneously in C-AFM on a P3HT thin film deposited on an ITO patterned glass substrate. A fiber-like morphology is clearly observed in both topographic and electrical images. Note the presence of a P3HT-free zone near the ITO electrode (marked by arrows in Figures 1A and B). At this point, the glass/ITO substrate is exposed and a zero signal is measured in the current image, confirming that the current flow really originates from charge transport in the semiconductor polymer.
Interestingly, a similar current intensity is measured regardless of the distance between the probe and the ITO electrode, as illustrated by the current profile in Figure 1C (no significant decrease in current intensity is observed when moving away from the ITO electrode). This constant current signal indicates that no potential drop occurs across the P3HT film and at the electrode contact.

Figures 2A and B show images of the typical morphology obtained by drop-casting from a P3HT:CNT solution. Figure 2A shows a flat brown background composed of a dense, homogeneous layer of 15 nm-wide P3HT fibrils, as illustrated in the insets. A second layer of fibrils is then observed, appearing yellow/orange in Fig. 2A. The fibrils making up this second layer are oriented in the same way as the underlying fibrils, suggesting that there is interaction between the fibrils during the deposition process, leading to dense, homogeneously oriented fibrils. Figure 2A shows a curved carbon nanotube (appearing in clear in Fig. 2A and highlighted) and highlighted in dotted red in the inset of Fig. 2B. The fibrils surrounding the nanotube appear to be perpendicular to its axis, as illustrated in the inset of Fig. 2B. This particular arrangement of fibrils, perpendicular to the nanotube, may play a role in charge transport in these hybrid films.

Figure 3A shows a 3 x 3 µm² C-AFM current image of a P3HT:NTC film, with the DC polarization of the sample varying progressively from 500 to -750 mV along the axis. The color code is as follows: green for positive current values and red for negative current values, while black corresponds to zero current.
The current direction is the same in both the carbon nanotubes and the P3HT matrix. As expected, current intensity increases with the absolute value of the DC bias. Irrespective of polarity and polarization intensity, a higher current is measured in the CNTs than in the P3HT matrix. The I-V curves extracted from Figure 3A and plotted in Figure 3B for the two materials show a different profile. A linear dependence is observed in the case of CNTs (ohmic behavior), whereas an asymmetrical and non-linear behavior is observed for P3HT. These observations are consistent with the expected metallic properties of CNTs and the p-type semiconducting nature of P3HT.

The nanoscale morphology and electrical properties of P3HT hybrids in fibril form and carbon nanotubes have been studied by C-AFM. In addition to confirming the ability of P3HT to efficiently disperse CNTs, the morphological analysis shows the tendency of P3HT fibrils to form multilayers with a common orientation and to grow perpendicular to the CNT surface in order to maximize π-π interactions and optimize the photovoltaic conversion efficiency of this semiconducting polymer.

Conductive mode AFM provides valuable information on the electrical properties of semiconducting polymer hybrid films loaded with carbon nanotubes.

Electrical characterization also shows a local semiconducting response for P3HT and a metallic response for CNTs. AFM in conductive mode makes it possible to map the current distribution on individual nanofibers 15 nm wide. The use of C-AFM therefore opens up the prospect of effectively studying photocurrent generation following illumination of an active organic photovoltaic layer on the nanometer scale, i.e. exactly where photophysical phenomena actually occur.

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